VATICAN CITY — The Vatican opened part of its secret archives Monday to let historians review millions of diplomatic letters, private correspondence and other church documents to gain insight into how the Holy See dealt with the growing persecution of Jews before World War II.

Researchers said it could take months or years to study the contents of some 30,000 bundles of documents from the 1922-39 papacy of Pius XI, a span when the rise of Nazism, Fascism and Soviet-bloc communism gripped Europe.

The opening is part of the Vatican’s efforts to defend Pius’ successor, the wartime Pope Pius XII, against claims he did not do enough to save Jews from the Holocaust during the wars.

The Vatican insists Pius XII, who earlier served as a church diplomat in Germany and later Vatican secretary of state under Pius XI, used discreet diplomacy that saved thousands of Jews.

Archives officials said at midday that some 50 researchers had shown credentials to gain admittance, although some of the scholars came to consult material on earlier papacies.

“There was a bit of chaos,” said Alessandro Visani, a researcher in contemporary history at Rome’s La Sapienza university.

He hopes the files will reveal the views that Roman Catholic prelates privately held about Mussolini’s racial laws affecting Italy’s Jewish community.

One tantalizing question revolves around an encyclical that Pius XI commissioned to denounce racism and the violent nationalism of Germany. But he died before releasing it, and it has never been made public.

The encyclical was never published “in part because of his death and in part because it was judged to be inopportune politically,” Visani said.

A La Sapienza colleague, Emma Fattorini, told The Associated Press by telephone after looking at the material that there were few mentions of the unpublished encyclical.

“I was stunned,” she said. “We can’t find various versions” of drafts researchers expected to find. She raised the possibility that some material might have been removed from the archives before they were opened.

A German researcher in Rome, Lutz Klinkhammer, said he didn’t expect any major discoveries concerning relations between the Vatican and Nazi Germany because three years ago the Vatican made available documents from the offices of the papal nuncios in Berlin and Munich during Pius XI’s papacy.

“We are not expecting any document to give us a scoop,” Klinkhammer said.

Visani also doesn’t foresee any bombshell findings. “The facts are known, more or less. You look for details and shading,” he said.

The Rev. Giovanni Sale, an Italian historian at the Jesuit magazine Civilta Cattolica, which is close to the Vatican, expressed confidence the archives will yield evidence to “correct” suspicions of anti-Semitism surrounding Pius XII.

The archives will provide “a new beginning for a history without prejudice,” Sale told AP Television News.